The present invention relates to an optical cross-connect for connecting and switching optical paths of a plurality of optical signals in an optical communication system, and more particularly to an analog beam-steering free-space optical switch that is constructed using micro-machining technology.
The development of the xe2x80x9cInformation Societyxe2x80x9d is advancing the development and deployment of optical communication systems that enable high-capacity information transfer. To meet the increasing demands of communication, this type of optical communication system employs Wavelength Division Multiplexing (WDM) transmission technology in which signals of different wavelengths are superposed on a single optical fiber for transmission and reception. In addition to a Point-to-Point mode in which communication between two points is joined using optical multiplexer/demultiplexers, this transmission technology introduces an Add-Drop mode for adding and dropping specific wavelengths at relay stations.
In order to implement this type of communication system, a light source adapted for high-speed modulation, optical fiber for high-capacity transmission, broadband fiber amplifiers, and multi-channel wavelength filters are indispensable. Of these components, optical switches that can selectively switch optical signals of any wavelength from a plurality of input ports and connect the optical signals to prescribed output ports is an important key technology for flexibly handling constantly changing communication demands as well as for coping with failures in communication lines.
On the other hand, the conversion to all-optical communication in which optical signals are transmitted without conversion to electrical signals is being advanced as one way for the development of optical communication systems in order to realize lower optical communication costs, system simplification, and faster transmission rate. This communication method is directed to using, in a large-scale switch for setting optical paths, an all-optical OXC (Optical Cross-Connect) as an optical switch. The xe2x80x9call-optical connectionxe2x80x9d refers to the way of connecting optical paths without first converting light to electricity and then connecting the electrical transmission lines.
An all-optical optical switch requires from small-scale switches having one-input and two-output (1xc3x972) to large-scale switches having 1000xc3x971000 or more input and output ports.
FIG. 1(a) shows a small-scale optical switch (1xc3x972) of the prior art, and FIG. 1(b) shows an example of hierarchically assembled small-scale optical switches. The small-scale optical switch is constructed using a driving circuit for mechanical connection 14, made up by a solenoid coil 11 and a permanent magnet 15, to selectively connect one input-side optical fiber 12 to either one of two output-side optical fibers 13 (NTT, RandD, Vol. 48, No. 9: 1999, pp. 665-673).
In the figure, reference numerals 16 and 17 stand for a movable fiber on the input side and static fibers on the output side, respectively. All these fibers are contained in a ferrule 18. The input and output optical fibers are externally connected through optical connectors 20.
As shown in FIG. 1(b), when using this type of small-scale optical switch, an Nxc3x97M multi-input-multi-output optical switch can be constituted by hierarchically assembling a plurality of small-scale optical switches 104. Nevertheless, the hierarchically assembled switch is not suitable for a large-scale switch because optical loss increases as the number of levels in the hierarchical structure increases.
FIG. 2 shows an example of a large-scale all-optical optical switch of the prior art, FIG. 2(a) showing a schematic view of the optical switch, FIG. 2(b) showing an entire optical switch array, FIG. 2(c) showing the arrangement of optical devices that constitute a portion of the optical switch array, and FIG. 2(d) showing the constitution of each individual optical device.
The optical device shown in FIG. 2 is an example of a free-space optical cross-connect for realizing inter-fiber optical connections in which micro-actuators individually drive micromirror elements arrayed by means of MEMS technology.
The example of the prior-art optical switch shown in FIG. 2(a) is made up by input ports 19, output ports 20, and two optical switch arrays 2101 and 2102. Input ports 19 are constituted by input-side fiber array 15 that is made up by N optical fibers secured to a through-hole array (not shown in the figure) of capillary array 1701, and collimation lens array 1801. Output ports 20 are similarly constituted by: output-side fiber array 16 that is made up by M optical fibers secured to capillary array 1702, and lens array 1802.
In this device, two optical switch arrays 2101 and 2102 are constituted by two-dimensionally arranging optical switch elements (hereinbelow referred to as optical switches) 105 in matrix form, wherein the number of the optical switches corresponds to the number of input/output ports, as shown in FIGS. 2(b) and (c). Each optical switch 105 is composed of an optical device section and a micro-actuator. In FIGS. 2(c) and (d), only the optical device is shown, and the micro-actuator is not shown in the figure.
The optical device is made up by: micromirror 203; mirror frame 303 that surrounds and pivotally bears micromirror 203 to rotate (tilt) micromirror 203 around the Ry axis; and frame 703 that surrounds and pivotally bears mirror frame 303 to rotate mirror frame 303 around the Rx axis.
Under electrostatic driving torque generated by a micro-actuator (not shown in the figure), micromirror 203 is capable of both tilting around the Ry axis and tilting around the Rx axis by means of mirror frame 303 that is rotatable around the Rx axis. The optical device is thus driven by the biaxial electrostatic driving torque generated by a micro-actuator to enable steering with respect to two degrees of freedom (around the axes in the Rx and Ry directions in FIG. 2(d)). This mode of drive is hereinafter referred to as biaxial drive.
Micromirror 203 and mirror frame 303 are pivotally borne by hinge springs 503 and 603, respectively, and are thus pulled back by an elastic restoring force proportional to the angle of rotation (tilt angle). Micromirror 203 and mirror frame 303 are at rest at the angle of tilt at which the electrostatic driving torque and elastic restoring force are in equilibrium.
A laser beam incident on micromirror 203 can thus be reflected in any direction.
This switch array allows changing the optical paths of beams as described hereinbelow (analog beam steering).
As shown in FIG. 2(a), optical signals exit from input port 19 and are collimated by passage through lens array 1801. An optical signal that has been collimated is then incident on micromirror 203 of optical switch array 2101 that corresponds to the micro-lens in lens array 1801 through which the optical signal of interest has passed. The reflected beam is steered by the biaxial drive of micromirror 203 such that the reflected beam of the optical signal is directed in a prescribed direction. The optical signal, emerging from optical switch array 2101, is next incident on a prescribed micromirror of second optical switch array 2102. Biaxial drive of the micromirror of second optical switch array 2102 directs the reflected beam of the optical signal to an optical fiber of output port 20 to take out the optical signal.
FIG. 3 shows the details of the optical device of the optical switch, FIG. 3(a) being a plan view of an example of a biaxial-driven free-space optical switch of the optical device, and FIG. 3(b) being a plan view of the hinge springs.
Micromirror 203 is pivotally borne by mirror frame 303 by means of a pair of hinge springs 503, and this mirror frame 303 is similarly pivotally borne by outer frame 703 by means of a pair of hinge springs 603.
In this device, the rotational axis of micromirror 203 and the rotational axis of mirror frame 303 are set in mutually orthogonal directions, and a reflected beam is steered two-dimensionally by independently driving around the two axes. The hinge springs may be of any shape to obtain a prescribed stiffness, and in this example, a serpentine or continuous fanfold shape (refer to FIG. 3(b)) has been adopted.
Two sets of electrode pairs 903 and 1003 are arranged orthogonally on the surface area of substrate 1102 that confronts micromirror 203. The area is hereinbelow referred to as the electrode region. These electrodes 903 and 1003 have substantially square shapes, and in combination with the grounded optical device (micromirror 203), constitute an electrostatic driving micro-actuator. The application of electrostatic force, which is controlled by controlling the applied voltage, causes micromirror 203 and mirror frame 303 to rotate around their respective axes.
FIG. 4 shows the principles of operation of the electrostatic drive. FIG. 4(a) shows a sectional view of micromirror 203 in a state in which electrode pair 903 is at the ground potential and no torque is applied to micromirror 203. As was explained with reference to FIG. 2 and FIG. 3, micromirror 203 is borne by mirror frame 303 by way of hinge springs 503, and mirror frame 303 is attached to frame 703 by way of hinge springs 603.
FIG. 4(b) shows a sectional view of the state in which voltage is applied to electrode pair 903 to apply torque to micromirror 203 and cause micromirror 203 to tilt. FIG. 4(c) is a side view showing the state in which both mirror 203 and mirror frame 303 are tilted around the respective axes when voltages are applied to both of the orthogonally arranged two pairs of electrodes.
In another prior art technology, it is proposed that optical paths can be constituted in planar form on the optical cross-connect and the mirror rises when the control signal is ON and falls when the control signal is OFF (digital crossbar system). This optical cross-connect is advantageous because it can be operated by simple digital control, it facilitates positioning of the input and output optical fibers, and further, it facilitates integration. However, it is disadvantageous because Nxc3x97N connections necessitate N2 switch elements while analog beam steering necessitates 2N switch elements, and moreover, because large optical path differences result in an increase in loss. As a result, the application of this method to large-scale switch is considered problematic.
Basically, the communication capacity in an optical communication system is determined by the product of the transmission rate per channel and number of channels. It can therefore be presumed that there will be a growing demand for achieving a high transmission rate and increasing the number of channels. Research into large-scale all-optical optical switches is therefore anticipated.
Of large-scale all-optical optical switches, analog beam-steering free-space optical switches are regarded as promising for realizing a compact, low-cost large-scale switch capable of, to some degree, suppressing optical loss. Such optical switches have been announced by Lucent Technologies or "khgr" ros (Now owned by Nortel).
In an analog beam-steering free-space optical switch of large scale, the number of employed micromirrors is large and the angle of steering of the micromirror therefore needs increasing as the number of input/output ports increases. In order to increase the steering angle of the micromirror, it is required to extend the range of the angle within which controlled steering of a micromirror is permitted.
As another method of extending the range of steering control, it has been proposed to increase the distance between the input/output ports and mirrors and also the distance between the two optical switch arrays so that the length of the optical path through free space elongates. This method offers an advantage that mirror tilt of a small angle can achieve a desired change in the direction of the optical signal. The method, however, entails the problems that the attendant enlargement of the beam radius not only causes eclipse in the mirror which causes optical loss, but also results in an increase in size of the optical switches and the module overall.
Thus, ensuring sufficiently large mirror tilt angle is a key point in constructing a large-scale optical switch having small size and low loss.
In the case of using an electrostatic actuator to drive the micromirror, however, a problem has been that the mirror tilt angle is limited by the driving torque characteristic.
FIG. 5 shows a simple analysis model that takes as an example a single-axis rotating mirror driven by an electrostatic actuator. In this example, the driving torque generated by electrostatic force can be represented by the following formula.                     a        =                              r            ⁢                          xe2x80x83                        ⁢            θ                    =                                    (                                                d                                      sin                    ⁢                                          xe2x80x83                                        ⁢                    θ                                                  -                x                            )                        ⁢            θ                                              (        1        )                                E        =                              V            a                    =                      V                          {                                                (                                                            d                                              sin                        ⁢                                                  xe2x80x83                                                ⁢                        θ                                                              -                    x                                    )                                ⁢                θ                            }                                                          (        2        )                                          T          e                =                                            ∫              L1              L2                        ⁢                                          1                2                            ⁢                              ϵ                0                            ⁢                              WE                2                            ⁢              x              ⁢                              ⅆ                x                                              =                                    1              2                        ⁢                          ϵ              0                        ⁢                          WV              2                        ⁢                                          ∫                L1                L2                            ⁢                                                x                                                            {                                                                        (                                                                                    d                                                              sin                                ⁢                                                                  xe2x80x83                                                                ⁢                                θ                                                                                      -                            x                                                    )                                                ⁢                        θ                                            }                                        2                                                  ⁢                                  ⅆ                  x                                                                                        (        3        )            
Here, E is the electric field, V is the applied voltage, W is the electrode width (width in a direction perpendicular to the plane of the figure), xcex8 is the mirror tilt angle, L1 and L2 are the positions of the two ends of the electrode, xcex50 is the dielectric constant of a vacuum, d is the distance between the micromirror and the electrode when E=0, and xcex1 is the distance between a micromirror position x and the electrode when the mirror tilt angle is xcex8. In the following description, the distances a and d between the micromirror and the electrode are referred to as the xe2x80x9cair gap.xe2x80x9d
FIG. 6 shows the torque characteristic curve that represents the dependency of the electrostatic driving torque (Te) upon the mirror tilt angle. The curve was calculated based on the above formulas. In this example, calculations were carried out with the electrostatic width W set to 150 xcexcm, the electrode length L set to 150 xcexcm (L1=80 xcexcm and L2=230 xcexcm), the air gap d set to 50 xcexcm, and the applied voltage V set to 150 V.
With these settings, the hinge spring characteristic curve that represents the elastic restoring force Kxcex8 for a tilt angle xcex8 (where K is the stiffness of the hinge spring) is a straight line that passes through the origin and is tangent to the torque characteristic curve. The tilt angle at the point of contact xcex8MAX is the maximum steering angle within the range of tilt angles in which stable angle positioning is possible, as will be explained hereinbelow.
When the micromirror tilts and the hinge springs are deformed, an elastic restoring force Kxcex8 proportional to tilt angle acts on the micromirror, as shown by the hinge spring characteristic line or the restoring line (straight line) of FIG. 6.
The torque characteristic curve intersects the hinge spring characteristic line at two points if the applied voltage is low. We denote, of these two points of intersection, the tilt angle of the intersection at the larger tilt angle as xcex8H, and the tilt angle of the intersection at the smaller tilt angle as xcex8L. The micromirror attains dynamic equilibrium at the tilt position xcex8L.
A rise of the applied voltage V results in an upward shift of the torque characteristic curve. Thus, as the applied voltage V rises, the two intersection points approach each other with an increase in xcex8L and a decrease in xcex8H. A further rise of the applied voltage V causes the two intersection points to coincide with each other at xcex8L=xcex8H=xcex8MAX, i.e., the torque characteristic curve is brought into contact with the hinge spring characteristic line. FIG. 6 represents this situation.
The tilt angle xcex8MAX is thus the maximum value of xcex8L, i.e., the maximum steering angle within the range of tilt angles in which stable angle positioning is possible.
In the example with the above-described design conditions, when the stiffness of the hinge springs that bear micromirror 203 is set to 32.5xc3x9710xe2x88x9210 N-m/rad, the maximum value xcex8MAX of the mirror tilt angle for the applied voltage 150 V is 5.2 degrees. The maximum tilt angle of mirror frame 303 is of the same level when the stiffness of the hinge springs and the area and position of the electrodes are similarly set, and biaxial drive of the optical device for steering optical signals can thus be realized over the range of xc2x15.2 degrees.
As described in the foregoing explanation, an analog beam-steering free-space optical switch is required to extend the range of mirror steering xcex8MAX in order to be adaptive to a tendency toward scale-up of the switch while suppressing optical loss.
In a biaxial-drive optical device, the angular range xcex8MAX for angular positioning of micromirror 203 and mirror frame 303 is limited by restrictions arising from the torque characteristics of the device and elastic characteristics of the parts. Control of the angular positions of micromirror 203 and mirror frame 303 is thus performed within a range of tilt angles limited by these restrictions.
Thus, in order to extend the range of micromirror steering of an analog beam-steering free-space optical switch, the electrostatic actuator must be designed to extend the maximum controllable tilt angle xcex8MAX.
When the maximum steering angle (maximum controllable tilt angle) of an optical device is set larger, however, the air gap must be increased to avoid electrostatic breakdown caused by contact between the micromirror and the electrode at a large tilt angle. Increasing the air gap while keeping the driving voltage fixed, however, lowers the torque characteristic curve and causes a problem that the obtainable maximum tilt angle becomes smaller.
Since lowering the torque characteristic curve, however, has the advantage of relieving a sharp rise in torque at large steering angles, the maximum controllable tilt angle xcex8MAX can be extended if the stiffness of the hinge springs is designed to be soft. Reducing the stiffness of the hinge spring, however, brings about the risk of a break during processing, complicates handling of the components, and further, reduces the response speed of the optical device and leads to switching delay.
The easiness of manufacture and the fast response can be guaranteed despite increasing the air gap if the applied voltage is raised while maintaining the stiffness of the hinge springs above a prescribed fixed value. However, increasing the applied voltage brings about an sharp rise in driving torque with respect to the variation of the tilt angle xcex8. As a result, the sharp rise characteristic of driving torque causes a decrease of the maximum tilt angle xcex8MAX. A high-voltage drive may also cause a loss in circuit reliability. In large-scale switches in particular, in which severe conditions are imposed regarding the arrangement of electrode wiring, the high-voltage drive increases the probability of the occurrence of problems such as parasitic discharge in narrow gaps between wiring.
It is an object of the present invention to provide an analog beam-steering free-space optical switch that allows an improvement in the controllability of steering angles with respect to electrostatic driving torque and that is capable of steering an optical signal in an extended range of steering angle by means of a low-voltage drive. It is another object of the present invention to provide an analog beam-steering free-space optical switch that is suitable to large-scale switches and that offers small size, high speed, low cost, and high reliability.
To achieve the above-described objects, the present invention adopts the basic construction described below.
The optical switch according to the present invention is an optical switch for connecting and switching optical paths of a plurality of optical signals and includes a plurality of optical devices and electrostatic actuators for driving these devices.
The optical devices are each pivotally borne so as to allow rotation around a prescribed center of rotation. The electrostatic actuator includes a substrate that holds the optical devices and a plurality of driving electrodes mounted on the substrate.
An electrostatic voltage, when applied between the optical devices and driving electrodes, generates an electrostatic driving torque for causing the optical device to tilt with respect to the substrate around the center of rotation.
Control of the electrostatic driving torque allows control of the tilt of the optical device around the center of rotation and changes the direction of reflection of optical signals.
In the first optical switch of the present invention, a plurality of driving electrodes are arranged in a radial pattern with respect to the electrode center, and each driving electrode is formed in a shape in which the electrode width of a prescribed outer portion thereof with respect to the electrode center narrows with progression toward the outside. Here, the electrode center is the orthogonal projection of the center of rotation onto the substrate surface.
Since the width of the electrodes thus decreases with progression toward the outer circumference of the driving electrodes, the electrostatic driving torque that is applied to the outer circumferential portion of the micromirror is reduced compared to the prior art even when the same electrostatic driving voltage is applied. Thus, since a decrease in the electrostatic attractive force caused by narrowing of the width of the driving electrode compensates for an increase in the electrostatic attractive force caused by an increase in the tilt angle, a steep rise of the electrostatic driving torque in a large steering angle region is relaxed.
As a result, it becomes possible to set the maximum tilt angle xcex8MAX to a large value thereby enabling large tilt-angle control while preventing possible collisions between the micromirror and the substrate caused by an accidental increase in the tilt angle when operating in a large steering angle.
The optical device has a micromirror, a mirror frame arranged to surround the circumference of the micromirror, and a frame arranged to surround the circumference of the mirror frame. The micromirror is pivotally borne by the mirror frame such that the micromirror can rotate around a first rotational axis and such that an elastic restoring force acts against this rotation. The mirror frame is also pivotally borne by the frame such that the mirror frame can rotate around a second rotational axis and such that an elastic restoring force acts against this rotation.
The first rotational axis and the second rotational axis intersect each other on a plane parallel to the substrate. This point of intersection therefore makes the center of rotation of the optical device.
The plurality of driving electrodes can include a first electrode pair and a second electrode pair: a first electrode pair generates electrostatic driving torque for causing the micromirror to rotate around the first rotational axis; and a second electrode pair generates electrostatic driving torque for causing the mirror frame to rotate around the second rotational axis.
The two electrodes of the first electrode pair are arranged on both sides of a first orthogonal plane and the two electrodes of the second electrode pair are arranged on both sides of a second orthogonal plane. Here, the first orthogonal plane is a plane that includes the first rotational axis and that is perpendicular to the substrate. The second orthogonal plane is a plane that includes the second rotational axis and that is perpendicular to the substrate.
In the case of biaxial drive, the first rotational axis and the second rotational axis are arranged orthogonal to each other.
In the first optical switch, each of the driving electrodes is formed in a shape such that the electrode width of a prescribed inner portion with respect to the center of the electrode narrows with progression toward the inside.
As a result, in a biaxial-drive optical device, the pair of electrodes for rotating the micromirror and the pair of electrodes for rotating the mirror frame can each be extended into the vicinity of the electrode center without causing geometric interference. As a result, a large electrostatic driving torque can be generated with a low driving voltage, and the electrostatic driving torque can be set to a high value in the region of small steering angles.
The first optical switch can include an embodiment in which each of the driving electrodes is formed in a shape such that the electrode width of a prescribed inner portion with respect to the electrode center narrows with radially inward progression, and moreover, is formed in a shape such that the electrode width of a prescribed outer portion with respect to the electrode center narrows with radially outward progression.
In this embodiment, the electrostatic driving torque characteristic curve shifts upward as a whole because the driving electrodes are extended toward the inside with respect to the electrode center, but the electrostatic driving torque characteristic curve shifts downward because the electrode width of the driving electrodes narrows with radially outward progression with respect to the electrode center. This downward shift is more conspicuous with larger tilt angles. Accordingly, in comparison with the prior art, the shape of the driving electrodes in this embodiment produces an electrostatic driving torque characteristic in which the electrostatic driving torque characteristic curve is shifted upward in the tilt-angle range where the tilt angle is small and the electrostatic driving torque characteristic curve is shifted downward in the tilt-angle range where the tilt angle is large. Thus, this electrostatic driving torque characteristic curve exhibits a characteristic that rises more gradually with increase in the tilt angle than the characteristic curve of the prior art.
As a technical advantage of this embodiment, the range, within which the tilt angle can be controlled, extends, i.e., the maximum controllable tilt angle increases, as described below:
As previously explained, within the range in which tilt angles are not particularly large, the electrostatic driving torque characteristic curve for the tilt angle of the present embodiment is shifted up from that of the prior art. As a result, the slope or gradient of a tangent line that is drawn from the origin to the electrostatic driving torque characteristic curve is larger than the slope of a tangent line in the prior art;
generally, when a fixed voltage is applied between the micromirror and driving electrodes to generate an electrostatic driving torque, this torque increases monotonically with increase in the tilt angle of the micromirror. However, the rate of change of this torque increases as the tilt angle increases (i.e., the electrostatic driving torque characteristic curve rises more sharply with increase in the tilt angle);
accordingly, even if the gradient of the electrostatic driving torque characteristic curve of the present embodiment were exactly the same as that of the prior art, the tilt angle coordinates in the point of contact of the tangent line would be larger in the present embodiment than in the prior art, because the gradient of the torque characteristic curve at the point of contact of the tangent line is larger in this embodiment than in the prior art; and
since the tilt angle coordinate at the point of contact of the tangent line represents the maximum controllable steering angle, as explained above, the electrode configuration of this embodiment causes the maximum controllable tilt angle to be increased.
This embodiment offers an advantage of providing a shorter response time than the prior art technique as well, because the slope of the tangent line, drawn from the origin to the electrostatic driving torque characteristic curve, corresponds to an elastic constant of the elastic member. Since the slope of the tangent line is larger in the present embodiment than in the prior art, the present embodiment enables to employ an elastic member of a high elastic constant, which causes a short response time.
In this way, it can be achieved to extend the range of steering angles, employing low-voltage drive while increasing the elastic constant of the elastic member of the pivotal bearing portion (for example, a hinge spring). A compact, low-cost, and highly reliable high-speed optical switch applicable to large-scale switches can thus be provided.
As one working example, the driving electrodes can each be formed in a rhombus shape. The shape of each driving electrode may also be formed as an oval shape having its major axis aligned with a radial direction relative to the electrode center.
An optical switch may be constituted by arranging optical devices and electrostatic actuators, as described in the foregoing explanation, in an array depending on the number of input and output ports.
The second embodiment of the optical switch of the present invention is an optical switch for connecting and switching optical paths of a plurality of optical signals and includes a plurality of optical devices and electrostatic actuators for driving these optical devices, wherein the optical devices are pivotally borne to allow rotation around a prescribed center of rotation.
Each of the optical devices is provided with: a micromirror, a mirror frame that is arranged to surround the circumference of the micromirror, and a frame arranged to surround the circumference of the mirror frame. The micromirror is pivotally borne by the mirror frame by means of elastic members such as hinge springs such that the micromirror can rotate around a first rotational axis and such that elastic restoring force acts against the rotation.
The mirror frame is pivotally borne by the frame such that the mirror frame can rotate around a second rotational axis and such that elastic restoring force acts against the rotation. The first rotational axis and the second rotational axis intersect each other in the plane that is parallel to the substrate, the point of intersection making the center of rotation of the optical device.
The electrostatic actuator includes the substrate that holds the optical device and a plurality of driving electrodes mounted on the substrate. The plurality of driving electrodes include a first electrode pair for generating electrostatic driving torque for causing the micromirror to rotate around the first rotational axis, and a second electrode pair for generating electrostatic driving torque for causing the mirror frame to rotate around the second rotational axis. Electrostatic voltages, when applied between the micromirror and the first and second driving electrodes, generate electrostatic driving torque for causing the micromirror to tilt with respect to the substrate around the center of rotation. Connecting and switching of optical paths is thus realized by changing the direction of reflection of optical signals.
In this embodiment, the first electrode pair is a pair of circular-segment electrodes of the same size having a central angle of approximately 180 degrees, the two electrodes being arranged concentric with the electrode center and confronting each other on both sides of a first orthogonal plane, this first orthogonal plane including the first rotational axis and extending perpendicular to the substrate surface. The second electrode pair is a pair of concentric circular-segment electrodes of the same size having a central angle of approximately 180 degrees. The two electrodes are arranged concentric with the electrode center and confronting each other on both sides of a second orthogonal plane, the second orthogonal plane including the second rotational axis and extending perpendicular to the substrate surface. The first electrode pair is arranged concentric with the second electrode pair and inside the second electrode pair with respect to the electrode center.
Wiring for supplying a power supply to each electrode of the first electrode pair is provided in the gap that extends between the confronting ends of the second electrode pair.
The micromirror of the optical device is preferably pivotally borne by the first hinge springs on the first rotational axis, and the mirror frame is preferably pivotally borne by the second hinge springs on the second rotational axis. In this case, the tilt stiffness of the second hinge springs is set stiffer than the tilt stiffness of first hinge springs.
Further, the ratio of the tilt stiffness of the first hinge springs to the tilt stiffness of the second hinge springs is preferably set to equal the ratio of the electrostatic driving torque to be generated by the first electrode pair to the electrostatic driving torque to be generated by the second electrode pair when the same driving voltage is applied between the first and second driving electrode pairs and the micromirror.
An optical switch is formed by arranging optical devices and electrostatic actuators depending on the number of input and output ports in an array form.
The above-described electrode configuration of an optical switch enables one pair of electrodes for rotating a micromirror and one pair of electrodes for rotating a mirror frame to be efficiently arranged to cover the largest possible electrode area in an effective electrode region on the substrate that confronts the micromirror.
In addition, the circular-segment electrode shape, in which the essential electrode width narrows toward an outward direction with respect to the electrode center, i.e., toward the outer circumference of the above-described effective electrode region, allows the electrostatic driving torque to be set to high values in the region of small steering angles, as with the optical switch of the above-described first embodiment; and further, enables suppression of too strong electrostatic force generated at tilt angle positions at which the micromirror and the electrodes approach, thereby allowing a suppression of steep rises in electrostatic driving torque.
In this case, taking into account the fact that the area of the electrodes for driving the mirror frame arranged in the outer circumference of the electrodes for driving the micromirror is larger than the area of the latter electrodes, the tilt stiffness of the hinge springs for pivotally bearing the mirror frame is set to be stiffer than the tilt stiffness of the hinge springs for pivotally bearing the micromirror in order to realize proper performance of steering control of the micromirror. This structure has the advantage of avoiding coupled tilt movement of the micromirror and mirror frame and therefore enables an improvement of the control of the micromirror.
Thus, as with the optical switch of the previously described first embodiment, this embodiment enables an extension of the range of the steering angle that is controllable under low-voltage drive while improving the stiffness of the hinge springs of the pivotal bearing portions, thereby enabling to provide an accurate optical switch applicable to large-scale switches.
The above and other objects, features, and advantages of the present invention will become apparent from the following description referring to the accompanying drawings, which illustrate examples of preferred embodiments of the present invention.